This invention relates generally to computed tomography (CT) imaging and, more particularly, to methods and apparatus for calibration of z-axis tracking loops for positioning a CT x-ray beam of a multi-slice CT imaging system.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two-dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
In a multi-slice system, movement of an x-ray beam penumbra over detector elements having dissimilar response functions can cause signal changes resulting in image artifacts. Opening system collimation to keep detector elements in the x-ray beam umbra can prevent artifacts but increases patient dosage. Known CT systems utilize a closed-loop z-axis tracking system to position the x-ray beam relative to a detector array. It would be desirable to provide improved methods and apparatus for calibration of such systems. In particular, it would be desirable to provide improved methods and apparatus for determining calibration parameters such as: (1) a target beam position at which to maintain the x-ray beam; (2) a transfer function to convert sensed tracking information into a beam position in millimeters; and (3) valid limits of the transfer function.